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Concepts on soil and soiling• Soil deposits on textile materials are usually of complex composition, • containing both oily and particulate matter. • Particulate matter - microscopic solid or liquid matter suspended in the
Earth's atmosphere• Particulate soil may be clay, soot or metal oxides. • Street dirt contains several components such as peat moss, cement,
silica, Raolin clay, mineral oil, carbon black and iron oxide. Oily soils are typically fatty materials secreted by the body. Most common within this category is shirt collar dirt consisting of skin cells, sebum and eccrine sweat.
• Various other substances that contribute to the soiling of textiles include stains from oily liquids and food residues.
• Soot -mass of impure carbon particles resulting from the incomplete combustion of hydrocarbons
• Clay is a fine-grained natural rock or soil material that combines one or more clay mineral with traces of metal oxides and organic matter.
• Peat moss – one kind of soil conditioner • a product which is added to soil to improve
the soil’s physical qualities, especially its ability to provide nutrition for plants
• Carbon black - produced by the incomplete combustion of heavy petroleum
• Oily soils are typically fatty materials secreted by the body. Most common within this category is shirt collar dirt consisting of skin cells, sebum and eccrine sweat.
• Sebum is a naturally occuring substance that is produced by the sebaceous glands, which are found in the skin and mucus membranes of mammals. The primary function of sebum is to moisturize, lubricate and protect skin and hair.
• Various other substances that contribute to the soiling of textiles include stains from oily liquids and food residues.
• A stain is a discoloration that can be clearly distinguished from the surface, material, or medium it is found upon. They are caused by the chemical or physical interaction of two dissimilar materials.
• The mechanism of soiling is influenced by various factors associated with the transport and adsorption of soil onto a textile.
• Electrostatic attractive forces are primarily responsible for the deposition of air-borne particulate soil onto curtains or upholstery.
• In contrast, shoe dirt ground into a carpet surface, or soiling of a shirt collar or sofa by oily soil, arises from direct contact, exacerbated by rubbing action.
• Finally cross transfer of soil or redeposition on the same fabric may take place during laundering of textile items. This last mechanism of soil transfer is referred to as wet soiling.
• Airborne particulate matter, which includes dust, dirt, soot, smoke, and liquid droplets emitted into the air, is small enough to be suspended in the atmosphere.
• Airborne particulates may be a complex mixture of organic and inorganic substances. They can be characterized by their physical attributes, which influence their transport and deposition, and their chemical composition, which influences their effect on health.
• Irrespective of the method of soil transfer onto a textile, the main cause of soiling is considered to be due to subsequent adhesion of the soil onto the fibre surface.
• This is effected mostly by van der Waals forces, which operate only over very short distances.
• The strength of this interactive force will depend on 1. The nature of the soil and of the substrate, 2. The area of contact between the two. Example: Kissa [71] has shown that iron oxide adsorption on• polyester and cotton fabric increases with greater applied
pressure; the soil particle is considered to deform under pressure and conform to the shape of the textile surface.
Factors influencing SoilingMoisture regain Natural fibers and regenerated cellulose rayons have high moisture regain and have little
tendency to accumulate static electricity. Even if static electricity is generated, it is quickly dissipated to the atmosphere. Therefore, the problem of soiling and soil removal is not very acute in the case of fibers having high moisture regain.
Synthetic fibers have low moisture regain, therefore they accumulate static electricity which attracts dirt and dust from atmosphere.
When the moisture regain of the fibers drops below 4%, soiling increases rapidly. Polyester has the lowest moisture regain (0.4%) among synthetic fibers; therefore it attracts
maximum soil. Since these fibers are hydrophobic, they do not swell in water and the removal of soil from the fiber becomes difficult.
In the case of blends with cellulosic fibers, whatever soil is removed from the cellulosic component during washing, gets redeposited on the synthetic fiber because the synthetic fiber being oleophilic, attracts oily matter from the dirty wash waters.
• Electrostatic chargeSynthetic fibers accumulate static charge during manufacture and during wear. Charged fibers attract soil from the atmosphere, positively charged fabric attracting more soil than the negatively charged one.
Fabric StructureFabric construction, yarn count, twist and the cross section of the fiber influence soiling.
• Smaller the denier, greater is the tendency to soil. A circular cross sectional fiber retains less soil than one with an irregular cross section. Higher the twist in the yarn, greater the soil retention. Fabric with protruding fibers assists soiling. Loosely woven and open knitted fabrics are more prone to soiling than closely woven fabrics but removal of soil from loosely woven fabrics is easy. Fabrics made from filament yarn do not get soiled as fast as those made from spun yarns.
Particle size of SoilThe smaller the size of the soil particles, grater is the soil retention by the fabric.
Electron microscopy techniques have shown oily soil to be located within the interfibre capillaries of yarns, on the surface of the yarn and embedded in crevices formed between adjacent fibres within the yarn.
Additionally, with cotton fibres soil may be located within the lumen, crenulations and secondary wall of the fibre.
Particularly in the case of damaged cotton, it is known that through repeated laundering and wear cycles sebum can wick rapidly into the fine capillaries of the fibre and into the cotton lumen. This degree of soiling, as found on shirt collars for instance, is especially difficult to remove during laundering.
• The success of any soil-release treatment is intimately associated with the removal of soil by detergent action.
• Detergency• physico-chcmical action of cleaning a solid surface by
an aqueous Solution of surfactants .lt is accomplished by a combination of effects involving the
1. alteration of interfacial tensions and 2. emulsification of the removal of soil.
SOIL-RELEASE CHEMISTRY
Carboxy-based finishesThe composition of this finish is based acrylic and methacrylic acid and ester
copolymers. An ester to acid ratio of 70:30 is typical. This ratio seems to provide the proper
blend of hydrophilicity and oleophobicity (hydrophilic-lipophilic balance, HLB) required for a soil release finish.
HLB values of about 15 are favored. Greater hydropilicity would strongly reduce durability to laundering. The ease
of incorporating different acrylic monomers into copolymers has led to a wide variety of available finishes.
other carboxy polymers that have been used as soil-release finishes include styrene-maleic anhydride copolymers and sodium carboxymethyl cellulose.
• Hydroxy-based finishesOne of the earliest soil-release materials was starch, Other starch- and cellulose based products that have been used as soil release agents include methyl cellulose, ethyl cellulose, hydroxypropyl starch, hydroxyethyl cellulose, hydroxypropylmethyl cellulose and hydrolyzed cellulose acetates.
• With some expectations these finished lack the laundering durability desired in finish expected to last of a garment and must be applied in combination with a binder or cross-linking agent.
• Ethoxy-based finishes
One important group of soil-release agents for polyester fibers is based on condensation copolymers of terephthalic acid with ethylene glycol and polyethylene glycol. The structure of this polyester –ether copolymer contains blocks of polyethylene terephthalate and polyxyethylene terephthalate that provide a structure that has regions of hydrophilicity interspersed with hydrophobic regions that have a strong attraction for the polyester surface.
• These products can provide extremely durable soil-release properties for polyester fabrics by either exhaust or pad applications with about 0.5% solids add-on. It is possible to exhaust apply these products during the dyeing process.
• A modification of the condensation copolymer compounds involves incorporating anionic character into the polymer chain by use of sulfonated monomers. High soil-release performance, excellent softness and combinability with fluorocarbon finishes may be achieved by special silicone/polyalkylene oxide copolymers
• Fluorine-based finishes
These unique polymers have the unusual property of being hydrophobic and oleophobhic in air and hydrophilic and oil-releasing during laundering process. This is called ‘dual-action’ mechanism. The hydrophilic blocks are shielded by the fluorocarbon segments when dry, presenting a repellent surface. After immersion in the wash bath, the hydrophilic blocks can swell and actually reverse the interfacial characteristics of the surface, yielding the hydrophilic surface necessary for oily soil release. Typically, these modified fluoropolymers are pad applied to fabrics in combination with the durable press crosslinking agents to increase the durability of the finish. The higher cost of the fluorochemical soil release agents compared to the acrylic copolymers is somewhat compensated by the low add-on required for soil-release performance. Mixtures of both polymers types provide a common compromise between efficiency and costs.
